A digital image used to process an image as digital data can be easily copied by a computer or the like and transmitted via a communication line without degrading the image quality, compared to a conventional analog image. This feature, however, makes it easy to illicitly copy and redistribute a digital image having a copyright or the like.
To prevent this, there is known a digital watermark method. Digital watermarks are roughly classified into an invisible digital watermark obtained by invisibly embedding watermark information such as copyright information or user information, and a visible digital watermark obtained by visibly forming in an image a watermark image such as the logotype of a company having a copyright.
As for the invisible digital watermark, embedded watermark information cannot be recognized or is hardly recognized in an embedded image at a glance. Watermark information is rarely deleted, but is illicitly copied and distributed more frequently than visible watermark information. Even if a digital image is illicitly copied or distributed, watermark information remains in the digital image. An illicit user can be specified by a user ID or the like embedded as the watermark information.
As for the visible digital watermark, watermark information is visibly written in a digital image. It is difficult to directly utilize the digital image, suppressing illicit copying and illicit distribution. As a conventional visible digital watermark embedding method, the pixel value of an image representing copyright information such as the logotype of a copyright holder is replaced with the pixel value of an original image, embedding copyright information in the original image. The drawback of this method is that the original image cannot be reconstructed without difference information because the pixel value of the original image is lost.
In online sales of a digital image via the Internet or the like, the copyright is protected by not fully disclosing digital contents before purchase. The user grasps rough contents of the image, and then purchases the image.
To achieve this purpose, a digital content distributor uses a method of disclosing or distributing a reduced image (thumbnail image) or an image prepared by intentionally degrading the image quality of all or part of an original image in order to present rough contents of the image to the customer.
One candidate of a means for degrading the image quality is a visible digital watermark containing the logotype of a copyright holder or the like. If the visible digital watermark is not reversible, the original image of a degraded-quality portion must be transmitted again. In this case, retransmission of only a key is safer than retransmission of the original image via a network in terms of the communication amount and tapping in retransmission. Online image delivery services require a method of removing image quality degradation by using a key.
One of the techniques is a semi-disclosure technique. According to the semi-disclosure technique, bit information at an arbitrary bit position is extracted in an arbitrary region of an original image, and subjected to arithmetic processing such as encryption based on a key, degrading the image quality of the original image.
In the use of the semi-disclosure technique, bit information of part of an original image is saved, and conversion processing based on a key is performed for an extracted bit string to control the image quality of an image to be disclosed. Inversion processing based on a key is done to reconstruct the original image from the quality-degraded image.
An example of the semi-disclosure technique is disclosed in Japanese Patent Laid-Open No. 8-256321. This reference proposes a method of extracting part of the bit string of an image which is compression-coded by JPEG or MPEG compression coding, directly converting the extracted bit string by an independently defined conversion method without referring to a part other than the extracted part, and decoding the image.
If the user wants to purchase the original image of a distributed image whose quality is degraded, he/she acquires a key used to convert the bit string. The user can acquire a completely reconstructed content from the partially scrambled image and the key.
This method preserves the feature of an original image, but is a kind of scramble (encryption). This method does not fully consider the image quality against noise added by conversion processing.
More specifically, the image quality degradation of an image to be converted (scrambled) is determined by only a uniformly extracted bit string set in advance regardless of the feature of the image or the like. This method does not consider the human visual characteristic which is less sensitive to a change in luminance at a high luminance value and sensitive to a change at a low luminance value. The image quality degradation is not uniform at any grayscale.
In other words, there is a demand for a method of adding proper noise which allows the image appreciator to satisfactorily confirm the outline of an image.